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authorLinus Torvalds <torvalds@linux-foundation.org>2012-05-04 14:46:02 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2012-05-04 14:46:02 -0700
commit2f624278626677bfaf73fef97f86b37981621f5c (patch)
tree011d5b7d5a6a74d476a80b513f3dd4d78762ed03
parentf0f376f204b6047bbb405180f796e93cc8444f09 (diff)
Fix __read_seqcount_begin() to use ACCESS_ONCE for sequence value read
We really need to use a ACCESS_ONCE() on the sequence value read in __read_seqcount_begin(), because otherwise the compiler might end up reloading the value in between the test and the return of it. As a result, it might end up returning an odd value (which means that a write is in progress). If the reader is then fast enough that that odd value is still the current one when the read_seqcount_retry() is done, we might end up with a "successful" read sequence, even despite the concurrent write being active. In practice this probably never really happens - there just isn't anything else going on around the read of the sequence count, and the common case is that we end up having a read barrier immediately afterwards. So the code sequence in which gcc might decide to reaload from memory is small, and there's no reason to believe it would ever actually do the reload. But if the compiler ever were to decide to do so, it would be incredibly annoying to debug. Let's just make sure. Cc: stable@kernel.org Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
-rw-r--r--include/linux/seqlock.h2
1 files changed, 1 insertions, 1 deletions
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index c6db9fb33c44..bb1fac5b8ee8 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -141,7 +141,7 @@ static inline unsigned __read_seqcount_begin(const seqcount_t *s)
unsigned ret;
repeat:
- ret = s->sequence;
+ ret = ACCESS_ONCE(s->sequence);
if (unlikely(ret & 1)) {
cpu_relax();
goto repeat;